The generator stator core is the largest monobloc component in a turbine-generator set. The stator core comprises thousands of thin steel laminations (also referred to as “punchings”) stacked horizontally and clamped together to form a cylindrical stator core disposed within a generator frame. Each lamination defines a central opening and thus when stacked an axial opening extends through the core. The laminations are held together by a plurality of axial through-bolts that extend from end-to-end through the core.
A rotor is disposed within the central opening and mechanically rotated by a rotating turbine. The rotor is responsive to an electrical current such that rotation generates electric current in stator windings. The stator current is supplied to a plurality of main and neutral electrical leads mounted to the generator frame then to electrical loads through a transmission and distribution system.
Core laminations or punchings are stacked vertically at the factory. After stacking is complete the core is maneuvered into a horizontal orientation or retained in the vertical orientation and loaded into the generator frame. In lieu of stacking individual punchings, a plurality of bonded laminations (referred to as a “donut”) can be vertically stacked to form the core.
Steady-state and transient forces generated during normal operation and transient conditions impose substantial forces on the stator core. These forces can distort the core geometric shape, cause the laminations to vibrate, and damage the core, rotor and/or frame. Also, mechanical fatigue caused by these forces can lead to premature failure of the generator.
To avoid these effects, the generator frame is fixed to a stable support such as the floor of a power plant and the stator core is solidly affixed to the generator frame. Two different attachment techniques and corresponding attachment components are employed to affix the core to the frame.
Keybars are used in one attachment technique. These long, axial members are disposed along an outer circumference of the stator core, specifically within slots along the outer circumference. The radially inwardly facing surface of each keybar is held within a slot by a geometrically capturing interfacing shape (for example a dovetail shape). A radially outwardly facing surface of each keybar is attached to the stator frame using various intermediate hardware components.
One such intermediate attachment component comprises a resilient spring bar. Several spring bars are distributed circumferentially around an interior surface of the frame and each spring bar extends axially through the frame. A first surface of each spring bar is attached to radially inwardly facing generator frame ribs and an opposing second surface of each spring bar is attached to a key bar mounting block or plate. The key bar block or plate is attached to the keybar. Thus the keybars are not attached directly to the generator frame but instead are attached through the spring bars.
The end of each keybar (both the exciter end and the turbine end) comprises a threaded portion for receiving a threaded nut and washer. The nuts are tightened to provide a clamping force to the stator core.
A second common attachment technique employs attachment hardware referred to as building bolts. A building bolt is a long, axial rod having one surface attached to the outer circumference of the stator core through a geometrically capturing interface, and an opposing surface welded to a plurality of generator frame ribs. Each building bolt includes a threaded portion at both the turbine end and the exciter end.
The building bolts are installed and attached to the frame ribs prior to insertion of the stator iron. The stator core iron (in the form of individual laminations also referred to as punchings or a plurality of bonded laminations referred to as donuts) is then stacked on the building bolts. A nut is tightened to provide a clamping force to the stator core.
FIG. 1 is a partial cutaway perspective view of a prior art electric generator 8 and a stator core 10 mounted within generator frame 12. Only certain pertinent components of the frame 12 are illustrated in FIG. 1. FIG. 1 further illustrates a plurality of spring bars 15 distributed around a circumference of the core 10. A first surface of each spring bar 15 is attached to a plurality of frame rings 13 by a fastener 19. The frame rings 13 are in turn welded to an inside surface of the generator frame 12.
Each spring bar 15 extends an axial length of the core 10. At a plurality of axially-distributed core locations a second surface of each spring bar 15 is attached to a key bracket or key block 20 using fasteners 18. Each key block 20 spans a width of a keybar 22 and a plurality of key blocks 20 are axially distributed along each keybar 22.
The keybars 22 are fixedly captured with the core 10 by a geometrically capturing interface with a groove defined in an outer surface of the core 10. The keybars 22 and the core grooves are shaped such that the keybars 22 are captured within the groove by the geometric capture feature, such as the illustrated dovetail shape. A fastener 7 is tightened to provide additional forces to secure the keybar 22 to the core 10. Thus the core 10 is connected to the generator frame by serial coupling of the keybars 22 geometrically retained within core grooves, the key blocks 20 and the spring bars 15.
Stator windings (also referred to as stator bars, but not illustrated) are disposed within winding slots 21. Through-bolts (not shown) extend axially through openings 23. The through-bolts and mating nuts (neither illustrated in FIG. 1) cooperate to exert inwardly-directed axial clamping forces on core end plates and the laminations that comprise the core 10.
As those skilled in the art are aware, several techniques and apparatuses are known for removing a stator core from a generator frame. According to one technique, first the rotor and the stator windings are removed. Where sufficient space and lifting capability are available (at the factory for example or at certain field locations), the generator frame is rotated into a vertical position using a crane or other hoisting device coupled to a lifting plate affixed to a first axial end of the core. Hydraulic jacks are then placed on a supported surface and extended vertically upwardly through the frame face to contact the core end plate at a second axial end of the core. By removing the fasteners 18, the keybars 22 are separated from the spring bars 15, separating the core 10 from the frame 24. The jacks are then activated to apply a vertically upward force to assist the crane or other hoisting device to vertically remove the core from the frame.
When vertically lifting the stator core, care must be taken to avoid damaging either the core or the generator frame by inadvertent contact during the lifting process. Given the relatively large diameter, axial length and weight of the stator core, and the relatively small radial clearance with the generator frame a small yaw of the stator core during the hoisting operation may cause an inadvertent impact with the frame. Cranes, hoists and similar heavy moving equipment are expensive to purchase or lease, require logistics planning to have them available on site when needed and further require skilled operating engineers.
To remove the core while maintained in a horizontal orientation, the rotor and stator windings are removed. The punchings (or donuts) are individually removed while the core-to-frame attachment components remain in place (i.e., the keybars or the building bolts), effectively piecewise simultaneously removing and disassembling the core from within the frame.
After the core is removed, a new core or a refurbished core is installed, again according to a piecewise process. First the punchings or donuts are individually installed on the key bars or building bolts. The remaining core iron components (i.e., interlaminar loading members, through bolts, end plates and finger plates (a steel structure that transfer the axial core clamping load directly to the stator punchings, the finger plates have a surface profile that mimics the surface profile of the punchings) are installed to complete the core.
Then the windings, including winding bars, winding slot components, parallel rings and end winding support structures are installed. This installation process is conventionally done with the core in a horizontal orientation.
As can be seen from this description, and as known by those skilled in the art, the piecewise removal and installation of a stator core is a time and labor intensive operation.